260 SENESCENCE AND REJUVENESCENCE 



increasing conductivity of the paths of correlation, even though 

 increase in size occurs, but in the lower organisms where the degree 

 of dominance is slight and conduction paths do not attain any high 

 degree of development, the decrease in rate of metabolism in the 

 dominant region which occurs with advancing senescence may 

 sooner or later bring about the physiological isolation of parts of 

 the individual, and reproduction and rejuvenescence result. Ex- 

 tended experimental and analytic investigation is necessary to 

 determine how far a natural physiological senescence and how far 

 incidental or external factors are concerned in particular cases, 

 but it must be borne in mind that the possibility of inducing and 

 controlling these reproductions with the aid of external conditions 

 does not in any way prove that they may not also be induced or 

 controlled by internal conditions quite independently of the 

 environment. 



Since this relation between senescence and reproduction unques- 

 tionably exists, it is evident that in the plants and lower animals 

 senescence must very frequently lead automatically to reproduction 

 and rejuvenescence in at least some parts of the previously existing 

 individual. In such cases senescence does not lead to death of the 

 whole, and often where the individual breaks up into separate cells 

 or fragments, death does not occur in any part. Instead of leading 

 inevitably to death, senescence in the lower organisms may itself 

 be a condition of reproduction and rejuvenescence and so of indefi- 

 nite continuation of life. 



CONCLUSION 



In the plants and lower animals the low degree of stability 

 of the protoplasmic substratum and the consequent low degree of 

 individuation make possible the frequent occurrence of agamic 

 reproduction. Since a greater or less degree of rejuvenescence is 

 associated with such reproduction, the process of individual senes- 

 cence may be more or less completely compensated in many cases 

 and the organism may appear not to grow old and may never reach 

 the death point. Often the decrease in metabolic rate with ad- 

 vancing senescence is the primary factor in bringing about physio- 

 logical isolation of parts, reproduction, and rejuvenescence, and in 



